The present invention relates to a press or pressure casting machine such as a die-cast machine and, more particularly, to a method of controlling a mold pressure pin for squeezing a molten material enclosed in a mold.
In general, in a pressure casting machine, when the molten material enclosed in the mold is solidified, its volume is contracted, causing a shrinkage porosity to be generated. As a result, the strength, the airtightness and the like are adversely effected. In particular, since a die-cast machine generates a steep temperature gradient, an excessively large shrinkage porosity is generated.
Hitherto, in order to prevent the generation of such shrinkage porosity, a pressure pin is used to squeeze a molten material enclosed in a mold so as to homogenize the structure.
As a method of controlling the pressure pin, a structure has been employed which is arranged in such a manner that an injection pressure detection sensor is used to detect the injection pressure. When the detected pressure has been raised to a predetermined pressure level, a solenoid valve for controlling the operation of the pressure pin is excited through a control panel so as to operate a pressure pin cylinder.
In the die-cast machine thus-constituted, the pressure pin must specially have a function of effectively squeezing the molten material by making the squeezing timing to be the most suitable timing because the molten material solidifies at a considerably high speed. That is, if the squeezing timing is too fast or too late, a satisfactory squeezing effect cannot be obtained. Furthermore, since the solidifying time is too short, the allowable range for the above-described timing will become extremely short.
According to the conventional technology, the timing at which the pressure pin is started has been determined only by using a signal transmitted from the injection pressure detection sensor of the die-cast machine. Therefore, it has been difficult to obtain the effective timing. Furthermore, the introduced depth of the pressure pin into the squeezed portion of the cast product has been measured for the comparison of the obtained result of the measurement with the stroke of the pressure pin so that the determination of the timing at which the pressure pin is started is changed.
However, since the relationship between the stroke of the pressure pin and the predetermined value of the timing at which the pressure pin is started is too complicated, the determination can be made only by trial and error and thereby an excessively long time has been required. In particular, since the predetermined timing must be varied to correspond to each mold and casting condition, an excessively tedious task must be performed.
Even if a value of the above-described timing is determined, the state of solidification of the molten material will be changed due to change in the temperature of the mold or the like. Furthermore, since the die-cast machine is also affected by the temperature of the hydraulic oil or the like, the relationship between the stroke of the pressure pin and the predetermined start timing can be deflected from the initial state. As a result, a problem arises in that the depth of the introduced pressure pin can be deviated from the allowable range. In order to administrate it, a great effort has been required.
It might be considered feasible to employ another method of determining the timing which is arranged in such a manner that a signal denoting the injection start or a position signal at the time of the high speed injection start is used to start a timer with which the solenoid valve for controlling the pressure pin in excited. However, similarly to such case where the injection pressure is used as the reference, it has been difficult to determine the timing at which the pressure pin is started and to stably maintain the timing.
In both the case in which the injection pressure is used as the reference and the case in which the injection start or the high speed injection start is used as the reference, the control signals are communicated with each other on the sequence of the control panel for controlling the operation of the die-cast machine. Therefore, the scan time of each of the relays and the like, which constitute the sequence, are added and thereby a problem arises in that dispersion cannot be reduced.